In lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plates, also called printing plate precursors, typically comprise an imageable layer applied over the surface of a hydrophilic substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material.
If after imaging, the imaged regions (the regions struck by imaging radiation) are removed in the developing process, revealing the underlying hydrophilic surface of the substrate, the plate is called a positive-working printing plate. Conversely, if the unimaged regions (the regions not struck by the imaging radiation) are removed by the developing process and the imaged regions remain, the plate is called a negative-working plate. In each instance, the regions of the radiation-sensitive layer that remain (i.e., the image areas) repel water and accept ink, and the regions of the hydrophilic surface revealed by the developing process accept water, typically a fountain solution.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque regions of the mask. The mask is usually a photographic negative of the desired image. If corrections are needed in the final image, a new mask must be made. This is a time-consuming process. In addition, the mask may change slightly in dimension due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging of imageable elements, which obviates the need for imaging through a negative, is becoming increasingly important in the printing industry. Multi-layer positive-working imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. These elements comprise at least two layers, an underlayer and an imageable layer, over a substrate with a hydrophilic surface. These elements are described, for example, in Shimazu, U.S. Pat. No. 6,294,311, and U.S. Pat. No. 6,352,812; Patel, U.S. Pat. No. 6,352,811; and Savariar-Hauck, U.S. Pat. No. 6,358,669, and U.S. Pat. No. 6,528,228; the disclosures of all of which are all incorporated herein by reference.
To obtain a printing plate with imagewise distribution of printable regions, it is necessary to remove the imaged regions of the imageable element by contacting the imaged imageable element with a suitable developer. High pH developers have been used for multi-layer positive-working imageable elements. High pH developers typically have a pH of at least about 11, more typically at least about 12, even more typically from about 12 to about 14. High pH developers also typically comprise at least one alkali metal silicate, such as lithium silicate, sodium silicate, and/or potassium silicate, and are typically substantially free of organic solvents. The alkalinity can be provided by using a hydroxide or an alkali metal silicate, or a mixture. Preferred hydroxides are ammonium, sodium, lithium and, especially, potassium hydroxides. The alkali metal silicate has a SiO2 to M2O weight ratio of at least 0.3 (where M is the alkali metal), preferably this ratio is from 0.3 to 1.2, more preferably 0.6 to 1.1, most preferably 0.7 to 1.0. The amount of alkali metal silicate in the developer is at least 20 g SiO2 per 100 g of composition and preferably from 20 to 80 g, most preferably it is from 40 to 65 g.
Because of their high pH, disposal of these developers without creating environmental problems can be difficult. In addition, these developers absorb carbon dioxide from the air causing their activity to change during use.
Spray-on processors, which rely on the force of the developer spray and the brushes and plushes to dislodge the imaged regions of the element, have typically been used. In these processors, the developer is sprayed onto the imaged imageable element, but the element is not immersed in the developer.
Thus, it is difficult to control the temperature of the developer.
Thus, a need exists for a method for preparing imaged multi-layer positive-working imageable elements that does not suffer from these disadvantages.